Electric power steering system

Abstract

A hydraulic power steering system is disclosed which has a steering control valve operatively connected to a steering shaft to be actuated by the torsional torque exerted thereon by a steering wheel, and a hydraulic cylinder operated by the control valve for assisting the steering of the vehicle. An electric motor drive unit is coupled to the steering shaft for exerting torque thereto in a direction for returning the steering wheel to a straight ahead position. An electronic control circuit is connected to the motor drive unit for exerting a variable amount of return torque to the steering shaft according to the steering angle of the steering shaft, or to both the steering angle and the travelling speed of the vehicle.

What is claimed is: 
     
       1. A power steering system for a motor vehicle, the system being of the type having a torque sensor operatively connected to a steering shaft for sensing torsional torque exerted on the steering shaft by a steering wheel, and assist power means for assisting steering of the vehicle in accordance with the torsional torque on the steering shaft, wherein the improvement comprises: (a) an actuator mounted on the steering shaft for exerting torque thereto in a direction for returning the steering wheel to a straight ahead position, the actuator being positioned closer to the steering wheel than the torque sensor;   (b) a steering angle sensor for sensing an angle through which the steering wheel is steered in either direction; and   (c) a control circuit connected between the steering angle sensor and the actuator to exert return torque to the steering shaft according to the steering angle of the steering shaft.   
     
     
       2. The power steering system of claim 1 further comprising a vehicle speed sensor for sensing the vehicle, the control circuit being connected to the vehicle speed for controlling the actuator to vary the return torque exerted on the steering shaft according to the vehicle speed. 
     
     
       3. The power steering system of claim 2 wherein the control circuit computes lateral acceleration from the output signals of the steering angle sensor and the vehicle speed sensor and controls the actuator according to the lateral acceleration so as to change the return torque in proportion to the lateral acceleration. 
     
     
       4. The power steering system of claim 1 wherein the control circuit computes the speed at which the steering wheel is steered from the output signal of the steering angle sensor and controls the actuator according to the steering speed so as to change the return torque in inverse proportional to the steering speed.